Semiconductor device



May 31, 1960 I F. H. MASTERSON 2,939,058

SEMICONDUCTOR DEVICE Original Filed Dec. 26, 1956 2 Sheets-Sheet 1 FIG.I 24 Iiim l FIG. 3

ATTORNEY May 31, 1960 F. H. MASTERSON 2,939,058

SEMICONDUCTOR DEVICE Original Filed Dec. 26, 1956 2 Sheets-Sheet 2 Unted States Patent Ofi" 2,939,058 Patented May 31, 1960 ICC SEMICONDUCTORDEVICE Frank H. Masterson, Poughkeepsie, N.Y., assignor to InternationalBusiness Machines Corporation, New York, N.Y., a corporation of New YorkDriginal application Dec. 26, 1956, Ser. No. 630,596. Divided and thisapplication Apr. 3, 1959, Ser. No. 804,032 1 Claims. (Cl. 317-436) Thisapplication is a division of application Serial No. 630,596, filedDecember 26, 1956, entitled Semiconductor Devices.

This invention relates to semiconductor devices and in particular totheautomatic fabrication of such devices as diodes and transistors in aglass encapsulated package.

It has been established in the art that a semiconductor device haslonger life and is more reliable when provided with a glass hermeticseal over the parts of the device responsible for its electricalcharacteristics. However, heretofore to in the art, due to problemsarising as a result of the small physical size and fragileness of thecomponent parts and the intense heat, a technique of automaticallyfabricating a glass encapsulated semiconductor device has not beenestablished. This invention is directed to the automatic fabrication ofa glass encapsulated semiconductor device wherein all elements of thedevice are assembled along a common axis and automatic machinefabrication techniques may be employed to assemble the device.

' A primary object of this invention is to provide a coaxial glassencapsulated semiconductor device.

Another object is to provide an automatic machine technique forfabricating a glass encapsulated semiconductor device.

1 'Another object is to provide acoaxial glass encapsulatedsemiconductor device.

Still another object is to provide an automatic machine fabricationtechniquefor fabricating a glass encapsulated semiconductor diode.

A related object is to provide a method of forming ohmic contacts tosemiconductor crystals.

Another related object is to provide a method of fusing glassthroughjthe use of an induction heater.

Still another, related object is to provide an improved glassencapsulated semiconductor diode structure.

Other objects of the invention will be pointed out in the followingdescription and claims and illustrated in theaccompanying drawings,which disclose, by way of example, the principle of the invention andthe best mode, which has been contemplated, of applying that principle.

In the drawings:

Figure 1 is a sectional view of a semiconductor diode illustrating theprinciples of this invention.

Figure 2 is a'schematic view of an assembly fixture illustrating themethod of bonding the semiconductor die to a supporting lead. V

Figure 3 is a schematic view of an assembly fixture capable of attachinga rectifying electrode to the semiconductor crystal.

Figure 4 is a schematic view of' an assembly fixture used in assemblinga semiconductor device according to this invention.

:Figure 5 is a sectional view along the lines X--X of the inductionheater, assembly and mounting holder of the fixture of Figure 4.

Referring now to Figure 1 for purposes of illustration, 3 glassencapsulated coaxial semiconductor diode is shown to point out theconstructional features of a semiconductor device made using thetechnique of this invention. The diode of Figure 1 comprises a crystalsupporting electrode 1 having a semiconductor crystal 2 ohmically bondedthereto through the use of a bonding wafer 3. A cathode electrode 4 isprovided having a cathode lead 5 forming a rectifying junction 6 withthe semiconductor crystal 2. Electrode 4 and 1 are maintained in fixedrelationship with respect to each other by a glass sleeve 7 which isfused to each of the electrodes 1 and 4 and forming an hermetic sealover the entire assembly. A plastic coating 8 has beenshown surroundingthe entire glass case for heat insulation, shock absorption and lightrestricting advantages that are well-known in the art. The diode ofFigure 1 is illustrative of the features of construction of thetechnique of this invention whereby semiconductor de vices may becoaxially assembled in operations, the critical nature of which iscontrolled by the nature of the materials used and the structure ofassembly equipment at each stage hence automatic machine fabricationtechniques are facilitated by virtue of the fact that delicate handoperation are avoided and the elements comprising the assembly aremounted along a common axis so that step by step assembly techniques canbe more readily performed.

In assembling a semiconductor device such as the diode of Figure 1, thefirst step of the process is to provide an ohmic connection between thesemiconductor crystal 2 and the supporting electrode 1. This isaccomplished in the technique of this invention by a step analogous toresistance welding and this step is illustrated by the schematicassembly fixture shown in Figure 2. In the fixture of Figure 2, asupporting element 10 is provided to retain the supporting electrode 1,the crystal 2 and the bonding wafer 3 during the bonding operation. Thesupporting element 10 is shown, for purposes of manufacturing the diodebeing illustrated, as having a recess 11 of sufiicient size and shape toaccommodate and retain one end of the crystal supporting electrode 1,the crystal 2 and the bonding Water 3 and having a hole therethrough toaccommodate the remainder of the supporting electrode 1. The holder 10is mounted on a frame 12 having a member extending above the holder suchthat a vertical member 13 may be so mounted as to apply downwardpressure to the combination of the crystal 2 and the bonding wafer 3 andthe supporting electrode 1 when mounted in the recess 11. A spring 14 isprovided to cause the member 13 to apply pressure to the elementsmounted in the holder 10. The purpose of the schematic fixture of Figure2 is to retain the supporting lead 1, the bonding wafer 3 and thecrystal 2 in electrical contact while current is passed therethrough forbonding. For this reason, the member 13 is equipped with a conductivetip 15 which is insulated by an insulating spacer 16. The conductive tip15 is equipped with an electrical connection 17 and the holder 10 ismade of conductive materials and is equipped with an electricalconnection 18.

In this step of the process the crystal supporting electrode 1 is firstassembled into the holder 10 and for purposes of illustration, thecrystal supporting electrode 1 in this case is shown as having a largerhead thereon to permit positioning in the recess 11 of the holder 10.This feature of construction is shown for illustration purposes only, itbeing understood that so long as the crystal supporting electrode is ofsufiicient size to provide a good mounting for the semiconductor crystal2, its actual size and head shape would be purely arbitrary. Further,

since the holder 10 serves merely to maintain the crystal suitable shapeto accomplish this purpose, for example, a chuck.

A bonding wafer 3 of a material capable of forming an eutectic alloywith the semiconductor crystal 2 and with the crystal supportingelectrode 1 is next inserted in the recess 11 in contact with theelectrode 1. The semiconductor crystal 2 is then inserted in the recess11 in contact with the wafer 3. The conductive tip 15 is then brought tobear on the crystal and pressure is maintained by the spring 14. Asource of power shown as the power supply 19 is applied between theelectrical connections 17 and 18 so that current flows in a series paththrough the electrode 1, the wafer 3 and the crystal 2. This serves toraise the temperature of these elements to the eutectic alloytemperature of the wafer 3-semiconductor crystal 2 system, and the wafer3-electrode 1 system. This technique is similar to the technique ofresistance welding in that a series electrical path is provided betweenthe output terminals of the power source 19, comprising the terminal 17,the tip 15, the crystal 2, the wafer 3, the electrode 1, the holder andthe terminal 18 and wherein the points of highest resistance in thiscurrent path are the interfaces between the crystal 2 and the wafer 3and between the wafer 3 and electrode 1. Under such conditions, thegreatest power dissipation and consequently the greatest heat generatedwill occur at the interfaces between the crystal 2 and the wafer 3 andbetween the water 3 and the electrode 1 and when the temperature atthese points due to this power dissipation reaches the eutectic alloytemperature of the wafer 3 and semiconductor crystal 2 system and theeutectic alloy temperature for the wafer 3 and electrode system 1. Theseelements will fuse together at a temperature lower than the meltingpoint of each of them and one single ohmic contact will be formed. Sinceit is characteristic of an eutectic alloy that the melting temperaturethereof is considerably lower than other melting points in the system,the control of temperature to make this connection is not critical.Further, such a bonding technique as is here described is of particularadvantage in semiconductor devices wherein PN junctions have been madeby diffusion since the time spent at high temperatures is short, noappreciable further diffusion takes place. The pressure holding elements1, 2 and 3 together is not critical nor are the materials from which theschematic fixture of Figure 2 is made, critical as long as the points ofhighest resistance in any series electrical path containing the crystal2, the wafer 3 and the electrode 1 occur at the interfaces between thecrystal 2 and the wafer 3, and between the wafer 3 and anode 1, and thepower applied is suificient to reach the highest melting eutectic alloytemperature of the systems comprising the wafer 3 and the crystal 2 andthe wafer 3 and the electrode 1.

While the choice of materials and the dimensions involved in such abonding operation as is illustrated in Figure 2 as above described mayvary widely, the relationship of the materials of the elements 1, 2, and3 must be such that both of the above described eutectic alloytemperatures are lower than the melting temperature of any of theelements 1, 2 or 3. The following materials and dimensions for theelements of the diode of Figure l have been included in order to providea proper perspective and to aid in practicing and understanding theinvention. It should be understood that such information is not to beconstrued as a limitation since it will be apparent to one skilled inthe art that a wide choice of specifications are available. Theelectrode 1 may be made for example of one of the materials used in theart for purposes of coelficient of expansion and for glass to metalsealing such as an alloy of 51% nickel and 49% iron or an alloy of 43%nickel and 57% iron sheated in borated copper or an alloy of 29% nickel,17% cobalt, 3% manganese and 51% iron, these alloys are known in the artas 52 Alloy, Dumet and Kovar respectively. The electrode 1 may have adiameter of .020 inch and have a shoulder on one end thereof having adiameter of .060 inch. The semiconductor crystal 2 may be made ofgermanium, for example of N type conductivity. The bonding wafer 3 maybe made of gold being .060 inch in diameter and having a thickness of.005 inch. The conductive tip 15 may be made of carbon, platinum ortungsten and the holder 10 is made of a high melting point material suchas those materials known in the art as Kanthal or Nichrome. The spring14 exerts 25 grams pressure per square centimeter and the power supply19 supplies l3 amperes at 3 volts for a period of 15 seconds. This isbelieved to create a temperature of about 450 C. at the bonding faces,namely, the faces between elements 2 and 3 and between 3 and 1, and thistemperature is higher than the eutectic alloy temperature of both thegold-germanium and the gold-dumct systems.

What has been described in connection with Figure 2 is a technique forapplying contacts to semiconductor crystals wherein the assemblyoperations performed are all accomplished along a single axis therebyfacilitating automatic machine fabrication and the bonding accomplishedis done in such a manner that the physical properties of the materialsemployed and the physical arrangement of the parts in the bondingoperation eliminates hand assembly operations and close control of suchfactors as temperature. It will be apparent to one skilled in the artthat by including appropriate conductivity directing impurities in thewafer 3 material electrical changes in the crystal 2 such as theformation of PN junctions may be accomplished in connection with thebonding operation.

The manner of applying small area rectifying electrodes to asemiconductor device using the technique of this invention is discussedfor the diode of Figure 1 in connection with the schematic fixture shownin Figure 3.

Referring now to Figure 3 a schematic fixture capable of attaching acathode wire 5 and forming a small area rectifying junction 6 with thesemiconductor crystal 2 is illustrated. The fixture of Figure 3comprises a head member 20 of conductive material capable of verticalmotion with respect to an anvil member 21 also of conductive material. Aguide member '22 is provided, mounted between the head 20 and the anvil21 and having therethrough a positioning and supporting orifice 23 ofinsulating material capable of guiding a small diameter wire shown aselement 5. Retaining devices shown as screws 24 and 25 respectively areprovided for purposes of positioning the elements to be assembled inthis operation and to insure good electrical contacts of the elementsbeing assembled with the members to which each is attached. Terminals 26and 27 are provided to permit the application of power from anappropriate power supply, such as element 19 in Figure 2, to be appliedbetween the element 5 and the semiconductor crystal 2 so as to form arectifying junction 6 at the point of contact of these elements.

In the formation of the rectifying connection, the subassembly formed inthe previous step, namely, the crystal supporting electrode 1 ohmicallybonded to the semiconductor crystal 2 through the wafer 3 is placed onthe anvil 21 with a portion of the electrode 1 extending through a holetherein and a good electrical contact between the crystal 2 and theanvil 21 is insured by tightening of the screw 25. The cathode lead 5 isinserted through a hole in the head 20 and through the insulatingorifice 23 until contact is made with a semiconductor crystal 2. Goodelectrical contact between the cathode lead 5 and the head 20 is insuredthrough a tightening of screw 24 and vertical motion may be imparted tothe cathode lead 5 sufficient to insure good bearing pressure of thecathode lead 5 on the semiconductor crystal 2 by movement of the head 20through the anvil 21. The application of electrical power betweenterminals 26 and 27 flows in the current path comprising terminal 27,head anvil 21 and the'terminal 26. In this instance, the point ofhighest resistance is at the point of bearing of the lead on the crystal2 and the greatest temperature generated in the current path is at thispoint of greatest power dissipation. Hence the lead 5 when made ofproper material with respect to the crystal 2, will form an eutecticalloyv with the semiconductor crystal 2 when the temperature at thispoint reaches the eutectic temperature. In order to insure a rectifyingor current multiplying contact appropriate conductivity directingimpurities are included in the material from which the cathodelead 5 isdrawn so that these impurities may be introduced into the crystal 2altering the conductivity thereof when the eutectic temperature isreached.

In this particular illustration for the diode of Figure 1. asatisfactory material for the cathode lead 5 has been found to be a goldor platinum-ruthenium wire .002 inch in diameter containing appropriateP type conductivity directing impurities such as indium. The power to beapplied between terminals 26 and 27 necessary to provide a satisfactoryrectifyingjunction 6 is 2 amperes at 20 volts for a period of 50milliseconds. At this point the semiconductor diode of Figure 1 iselectrically complete so that any testing or rectifying contactimprovement steps that are believed to be advantageous may be performedat thistime.

The final assembly and encapsulation operation of this inventionareaccomplished inconnection with the schematic piece of equipmentillustrated in Figure 4. In Figure 4 an assembly device is illustratedhaving a glass sleeve holder 30 capable of vertical motion and twoelectrode holders 31 and 32 each being capable of vertical motion andprovided with means to maintain them in fixed relationship with respectto each other. These features are shown in Figure 4 by the fact that theholders 30, 31 and 32 are slidably mounted on the frame 33 and mayberetained in position by screws 34, 34A and 3413. Induction heaters 34and 34A are provided and positioned so as to be able to fuse the ends ofa glass sleeve to be used as the sealing enclosure over thesemiconductor device being manufactured. The holder 30 is of appropriatesize to permit the ends of a glass sleeve such as the sleeve 7 of Figure1 to extend beyond the holder. Induction heater inserts 36 and 36A areprovided. between the induction heating coils and the glass sleeve 7for'purposes to be later explained.

In this step of the process the semiconductor device, for example, thediode of Figure 1 is formed and glass encapsulation thereof isaccomplished in the following general manner. The semiconductor diodesubassembly, comprising the supporting electrode 1, the wafer 3, asemiconductor crystal 2 and the cathode lead 5, are mounted in oneholder, for example, holder 32 of the assembly fixture of "Figure 4 andappropriately secured therein by means such as a screw 37. Holder 30 ispositioned over the diode subassembly as by releasing screw 34A, movingthe holder 30 and retightening screw 34A. A glass sleeve 7 ofappropriate dimensions for the semiconductor device being manufacturedis positioned in the holder 30 with both ends exposed. The holder 30 isthen moved toward holder 32 so that the sleeve 7 is positioned aroundthe crystal supporting electrode 1 and diode subassembly is exposed. Thecathode electrode 4 is placed in the holder 31 and rigidly securedtherein as by means such as a screw 38. Holder 31 is now moved in thedirection of holder 32 until the cathode lead 5 comes into contact withthe cathode electrode 4.

Referring now to Figure 5 an enlarged cross-sectional view of theassembly is shown wherein the cathode electrode 4 is shown incontactwith cathode lead 5 and appropriate spot welding equipment, not shown,is used to form a spot weld 9 between the electrode 4 and the lead 5.The technique of spot welding is well-known in the art and the equipmentfor its practice is readily available. Any. equipment capable of causingsufiicient electric power to be dissipated at the point of contact of ithe elements 4 and 5 will cause the melting necessary for spot welding.For purposes of illustration,v a shoulder has been shown on theelectrode 4, to facilitate positioning,

this feature is arbitrary and as long as an electrically low resistanceand high mechanical stability spot weld between the lead 5 and theelectrode 4 is acquired, the shoulder, while helpful due to the smallphysical size of the elements, is not necessary. ,Referring again toFigure 4, once the spot weld 9 has been made, the holders 31 and 32 aremoved toward each other a small distance. This is illustrated as'a bendin the lead 5. The function of the movement of electrodes 1 and 4 towardeach other is to compensate for a difference in coeflicient of expansionbetween the glass sleeve 7 and the elements making up the remainder ofthe assembly. .The holder 30 is now moved in the direction of element 31until the glass sleeve 7 surrounds and the ends are positioned equallydistant from the semiconductor diode assembly. The induction heaters 35and 35A then are energized to cause the glass of the sleeve 7 to fuseand form a hermetic seal with the leads 1 and 4.

Referring again to Figure 5, in this larger view, the elements 36 and36A are shown inserted between coils of the induction heaters35 and 35Aand the glass sleeve 7. The elements36 and 36A are made of electricallyconductive, high melting temperature materials which,

serve the function of radiating heat until the conductive temperature ofglass has been reached. Glass-has been found 'to be a conductorand-susceptible to induction heating, when the temperature of the glassreaches 900" C. and above, but glass is not a conductor until thistemperature is reached. For this purpose elements 36 and 36A ofconductive material are inserted inside the coils 34 and 35 to becomeheated as a result of the inductive energy applied thereto and toradiate such heat to the glass sleeve 7 until the conductive temperatureof glass is reached, at which time elements 36 and 36A may be removed ifdesired. As a result of this, it is possible to use induction heating tofuse the glass and thereby to avoid the corrosive effects and difiiculttemperature control associated with the use of flame, as a source ofheat. Upon the completion of the fusing step the semiconductor diode isas shown in Figure 1 may be provided with an insulating coating 8 havingthe heat insulating, shock resisting and light retarding advantageswell-known in the art. The coating 8 may be applied by an appropriatedipping or spraying technique.

What has been described is a technique of fabrication of semiconductordevices whereby all assembly operations are performed along a commonaxis and each assembly operation is performed in such a manner that thephysical properties of the elements being assembled and the generalstructural principles of the fixture in which the elements are assembledcooperate to avoid delicate hand operations and extremely close controlrequirements and a method of employing induction heating to a normallynonconductive material is utilized. The technique has been illustratedin connection with the fabrication of a semiconductor diode although aswill be apparent to one skilled in the art the principles of thetechnique are applicable to semiconductor devices other than diodes. Theassembly fixtures described have been limited in structural detail so asto illustrate only the features of construction necessary forexplanation and for the practice of the invention.

While there have been shown and described and pointed out thefundamental novel features of the invention as applied to a preferredembodiment, it will be understood that various omissions andsubstitutions and changes in the form and details of the deviceillustrated and in its operation may be made by those skilled in the artwithout departing from the spirit of the invention. It is the inten- 7tion therefore, to be limited only as indicated by the following claims.

What is claimed is:

1. A semiconductor diode comprising in combination a supportingelectrode, a semiconductor crystal, at wafer of metal capable of formingan eutectic alloy with said supporting electrode and with said crystalpositioned between and fused to said crystal and said supportingelectrode, a rectifying electrode containing appropriatesemiconductorconductivity directing impurities and capable of forming an eutecticalloy with said crystal fused to and forming a rectifying contact withsaid crystal, an external electrode welded to said rectifying electrodeat a point spaced from said crystal and glass sleeve surrounding saidcrystal fused to portions of said external and said supporting electroderemote from said crystal.

2. A semiconductor diode comprising in combination a first Dumet wire, asemiconductor crystal, a gold wafer positioned between and fused to saidcrystal and to one end of said first wire, a gold wire containingappropriate conductivity directing impurities fused at one end to saidcrystal, a second Dumet wire fused to said gold wire at a point spacedfrom said crystal and a glass sleeve surrounding said crystal and fusedto said first and second Dumet wires.

3. A semiconductor diode comprising in combination a first Kovar wire, asemiconductor crystal, a gold wafer positioned between and fused to saidcrystal and to one end of said first wire, a gold wire containingappropriate conductivity directing impurities fused at one end to saidcrystal, a second Kovar" wire fused to said gold wire at a point spacedfrom said crystal and a glass sleeve surrounding said crystal and fusedto said first and said second Kovar wires.

4. A semiconductor diode comprising in combination a first Dumet wire, asemiconductor crystal, a platinumruthenium wafer positioned between andfused to said crystal and to one end of said first wire, aplatinumruthenium wire containing appropriate conductivity directingimpurities fused at one end to said crystal, a second Dumet wire fusedto said platinum-ruthenium wire at a point spaced from said crystal anda glass sleeve surrounding said crystal and fused to said first and saidsecond Dumet wires.

5. A semiconductor diode comprising in combination a first Kovar wire, asemiconductor crystal, a platinumruthenium wafer positioned between andfused to said crystal and to one end of said first wire, aplatinumruthenium wire containing appropriate conductivity directingimpurities fused at one end to said crystal, a second Kovar wire fusedto said platinum-ruthenium wire at a point spaced from said crystal anda glass sleeve surrounding said crystal and fused to said first and saidsecond Kovar" wires.

References Cited in the file of this patent UNITED STATES PATENTS2,796,563 Ebers et al. June 18, 1957 2,854,612 Zaratkiewicz Sept. 30,1958 2,861,226 Lootens Nov. 18, 1958

